Cupid's arrow for snails

Helen Phillips

Like Cupid and his arrow, the courting snail fires a 'love dart' at its partner. This bizarre sexual ritual has itself courted many a theory about its function. One thing is certain - the archer is not aiming for the heart.

At last it seems the mystery is solved. In the latest issue of the Journal of Experimental Biology, Joris Koene and Ronald Chase of McGill University in Montreal, Canada, show that the dart transfers mucus, whose active ingredients trigger changes in the female reproductive organs, increasing the chance that the archer's sperm will fertilize eggs.

As snails are hermaphrodite - they have both male and female reproductive organs - it is even more difficult to work out the function of the dart. There have been many suggestions, including a role in signalling the readiness of the shooter to lay eggs or to mate, or that the dart might induce egg laying. It is not until after shooting the dart that the two snails try to intromit each other.

But mating can continue perfectly successfully when the dart misses, or if no darts are fired. Snails mate many times, and can only produce a dart every week or so, so inevitably some encounters will be dart-free. There are no obvious effects on arousal or mating behaviour with or without the dart, so it seems that any fundamental role in triggering sexual behaviour is unlikely.

The dart is made of calcium, which has led others to suggest that it might be a nuptial gift, providing an extra source of calcium for egg production. But Koene and Chase find that the dart is rarely internalized by the recipient, and little calcium could enter the body in this way. So what could it be for?

The pair believes the key to the love dart's function is in the tendency of snails to mate many times before fertilizing any eggs. Like insects, they have a special chamber in the female reproductive tract where sperm is collected and stored before fertilization. The competition to become a father takes place as much between sperms after mating as in finding a mate in the first place. Perhaps the dart could be helping in this process of sperm competition, suggest the researchers.

When the dart is expelled from its storage sac it is covered by a mucus layer, produced by a pair of glands. The dart seems to act as a hypodermic device, injecting mucus into the recipient. So the researchers tested what effect the mucus had on various internal organs of the snail.

They found that the mucus triggered muscular contractions in the reproductive system. Most contractions were short-lived, but there were two more significant and longer-lasting effects on the female reproductive organs. One was on the copulatory canal, and the other on the bursa tract, which leads to the chamber where many of the failed sperm are finally digested.

The contractions closed off the entrance to the bursa tract, and opened the way to the region where the completed spermatophore is deposited. From here the sperm can escape into the female tract. After a short delay, waves of contractions start in the region, which could help more sperm reach the sacs where fertilization takes place.

It is not clear what ingredient in the mucus is responsible for triggering the muscular activity, but it seems that the snails are not too fussy about what type of mucus they respond to. The mucus used to lubricate the foot is also able to trigger contractions, so it seems that the active substance is a general ingredient in snail mucus.

Injecting mucus lacks the romance of the story of Cupid's arrow, but it seems that, for snails, romance is not what is needed to improve the chances of becoming a father.